1. Field of the Invention
The present invention relates to a fuel cell including a membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly. The membrane electrode assembly includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The anode and the cathode include electrode catalyst layers provided respectively on both surfaces of the electrolyte membrane.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a polymer ion exchange membrane as an electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. Each of the anode and the cathode is made of an electrode catalyst layer and a gas diffusion layer (e.g., porous carbon). The membrane electrode assembly is sandwiched between separators (bipolar plates) to form a power generation cell. In use, generally, a predetermined number of power generation cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. A gas chiefly containing oxygen such as the air (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. The electrode catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In this type of the fuel cell, for example, the structure as disclosed in Japanese Laid-Open Patent Publication No. 2002-373678 is adopted. In the conventional technique, as shown in FIG. 7, a unit cell 1 includes an electrolyte membrane 2, catalyst electrodes 3a, 3b formed on both surfaces of the electrolyte membrane 2, and gas diffusion electrodes 4a, 4b formed on the catalyst electrodes 3a, 3b oppositely.
The gas diffusion electrodes 4a, 4b are sandwiched between separators 5a, 5b. A fuel gas flow field 6a for supplying a fuel gas to the catalyst electrode 3a is formed between the gas diffusion electrode 4a and the separator 5a, and an oxygen-containing gas flow field 6b for supplying an oxygen-containing gas to the catalyst electrode 3b is formed between the gas diffusion electrode 4b and the separator 5b. 
In the unit cell 1, at the time of power generation, water is likely to be produced at the catalyst electrode 3b on the cathode side, and area of the electrolyte membrane 2 to which the catalyst electrode 3b is applied is swelled. Therefore, a dimensional change may occur between the area of the electrolyte membrane 2 to which the catalyst electrodes 3a, 3b are applied, and the area of the electrolyte membrane 2 to which the catalyst electrodes 3a, 3b are applied. The dimensional change may cause stress generation undesirably. Further, edges of the catalyst electrodes 3a, 3b are in the outer boundary area to which the catalyst is applied. In the outer boundary area, the electrolyte membrane 2 may be damaged easily by the stress concentration.
Though the gas diffusion electrodes 4a, 4b are sandwiched by a plurality of protrusions 7a, 7b provided on the separators 5a, 5b, the edges of the catalyst electrodes 3a, 3b are not sandwiched reliably. Thus, in the conventional technique, cracks or the like may be generated in the electrolyte membrane 2 undesirably.